gatsby-node.js

const fs = require('fs');
const util = require('util');
const gm = require('gray-matter');

const remark = require('remark');

remark.mdx = require('remark-mdx');
remark.html = require('remark-html');

const readFile = util.promisify(fs.readFile);

exports.onCreatePage = async ({ page, actions }) => {
  if (!page.context.frontmatter || !/^\/blog\//.test(page.path)) return;

  const src = await readFile(page.component, 'utf8');
  const post = gm(src, {
    excerpt: (file) => {
      const [first] = file.content.split('\n\n');
      file.excerpt = first;
    },
  });

  const compiler = remark().use(remark.mdx).use(remark.html);

  const html = await compiler.process(post.content);
  const excerpt = await compiler.process(post.excerpt);

  const next = Object.assign({}, page, {
    context: {
      ...page.context,
      html: html.contents,
      excerpt: excerpt.contents,
    },
  });
  actions.deletePage(page);
  actions.createPage(next);
};

exports.createSchemaCustomization = ({ actions }) => {
  const { createTypes } = actions;
  const typeDefs = `
    type SitePage implements Node {
      context: Context
    }

    type Context  {
      frontmatter: Frontmatter
    }
    
    type Frontmatter {
      title: String
      date: Date
      draft: Boolean
      minRead: String
    }
  `;
  createTypes(typeDefs);
};